A spectral device utilizing an interference filter in which dielectric thin films having different refractive indices are alternately laminated and its intermediate cavity layer is formed of a dielectric thin film having an intermediate refractive index has been conventionally known (refer to the following Patent Document 1). This spectral device has an interference filter provided so as to be freely turnable and changes an incident angle to the interference filter when parallel light rays are made incident to the interference filter, so as to continuously change its transmissive wavelength.
As one having a similar configuration, a wavelength-variable filter in which its wavelength variability according to an incident angle is achieved by rotating a rotary table provided with dielectric multilayer filters and controlling an incident angle of parallel light to the dielectric multilayer filters, has been known (refer to the following Patent Document 2). This wavelength-variable filter achieves an output of transmitted light having a broader wavelength range by adopting a configuration in which four filters are disposed so as to be rotationally symmetric on the rotary table.